Spamming user+1@gmail, user+2@gmail takes absolutely no technical knowledge whatsoever - anyone can do it with 1 gmail account.
Spamming user1@domain, user2@domain etc requires 1 of two things:
you can sign up for multiple email accounts using a third party service. You’re going to run into trouble with Gmail or other big providers if you start creating accounts en masse.
you create your own email server. this requires someone with selfhosting knowledge and some basic coding (or rather server config) experience.